Off-hook detector with reduced sensitivity to common mode

Abstract

An off-hook detector for use in subscriber line interface circuits to indicate status of the subscriber line, or loop, exhibits reduced sensitivity to common mode or longitudinal currents. A central signal from the exchange switches the transmission bridge for the subscriber line from a balanced to an unbalanced condition. In the balanced condition a difference detector indicates loop status. In the unbalanced condition, a comparator compares the voltage on one side of the transmission bridge to a reference voltage to indicate loop status.

What is claimed is: 
     
       1. An off-hook detector for use in a subscriber line interface circuit to provide a loop signal to indicate the open-loop or closed-loop status of a subscriber line, comprising; a transmission bridge comprising a first terminal and a second terminal, a first branch connected between said first terminal and one side of a central battery, a second branch connected between said second terminal and the other side of said central battery, said branches normally being symmetrical;   means for introducing an asymmetry in said second branch in response to a control signal;   a difference detector having inputs connected to said two terminals and an output that supplies a first logic signal the state of which indicates the open-loop or closed-loop status of the subscriber line;   a comparator having a first input connected via a low pass filter to said second terminal, a second input connected to a voltage threshold, and an output that produces a second logic signal the state of which indicates the open-loop or closed-loop status of the subscriber line;   a logic circuit connected to said difference detector and said comparator and responsive to said control signal to provide said first logic signal as the loop signal when said branches are symmetrical and to provide said second logic signal as the loop signal when an asymmetry is introduced in response to said control signal.